Remove net451 target framework from Xamarin.MacDev.csproj

VS projects are now compatible with .NET 4.6.1 so we don't need MacDev.csproj to target 4.5.1 anymore

Xamarin.MacDev/Xamarin.MacDev.csproj

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>net451;net461;netstandard2.0</TargetFrameworks>
+    <TargetFrameworks>net461;netstandard2.0</TargetFrameworks>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
     <LangVersion>latest</LangVersion>